The present invention relates generally to the field of digitally controlled ink jet printing systems. It particularly relates to improving those systems that utilize continuous ink streams, whether the systems are heated. One such system uses heat to deflect the stream""s flow between a non-print mode and a print mode.
Ink jet printing is only one of many digitally controlled printing systems. Other digital printing systems include laser electrophotographic printers, LED electrophotographic printers, dot matrix impact printers, thermal paper printers, film recorders, thermal wax printers, and dye diffusion thermal transfer printers. Ink jet printers have become distinguished from the other digital printing systems because of their non-impact nature, low noise, use of plain paper, and avoidance of toner transfers and filing.
Ink jet printers can be categorized as either drop-on-demand or continuous systems. Major developments in continuous ink jet printing are as follows:
Continuous ink jet printing itself dates back to at least 1929. See U.S. Pat. No. 1,941,001, which issued to Hansell that year.
U.S. Pat. No. 3,373,437, which issued to Sweet et al. in March 1968, discloses an array of continuous ink jet nozzles wherein ink drops to be printed are selectively charged and deflected towards the recording medium. This technique is known as binary deflection continuous ink jet printing, and is used by several manufacturers, including Elmjet and Scitex.
U.S. Pat. No. 3,416,153 issued to Hertz et al. in December 1968. It discloses a method of achieving variable optical density of printed spots, in continuous ink jet printing. Therein the electrostatic dispersion of a charged drop stream serves to modulate the number of droplets, which pass through a small aperture. This technique is used in ink jet printers manufactured by Iris.
U.S. Pat. No. 4,346,387 issued to Hertz in 1982, discloses a method and apparatus for controlling the electrostatic charge on droplets. The droplets are formed by the breaking up of a pressurized liquid stream, at a drop formation point located within an electrostatic charging tunnel, having an electrical field. Drop formation is effected at a point in the electric field, corresponding to whatever predetermined charge is desired. In addition to charging tunnels, deflection plates are used to actually deflect the drops.
Until recently, conventional continuous ink jet techniques all utilized, in one form or another, electrostatic charging tunnels that were placed close to the point where the drops are formed in a stream. In the tunnels, individual drops may be charged selectively. The selected drops are charged and deflected downstream by the presence of deflector plates that have a large potential difference between them. A gutter (sometimes referred to as a xe2x80x9ccatcherxe2x80x9d) is normally used to intercept the charged drops and establish a non-print mode, while the uncharged drops are free to strike the recording medium in a print mode as the ink stream is thereby deflected, between the xe2x80x9cnon-printxe2x80x9d mode and the xe2x80x9cprintxe2x80x9d mode. The electrostatically charged non-printed drops are passed from the gutter to collection bottles and recycled.
Recently, a novel continuous ink jet printer system has been developed which renders the above-described electrostatic charging tunnels unnecessary. Additionally, it serves to better separate the functions of (1) droplet formation and (2) droplet deflection. That system is disclosed in our recently issued U.S. Pat. No. 6,079,821 entitled xe2x80x9cCONTINUOUS INK JET PRINTER WITH ASYMMETRIC HEATING DROP DEFLECTIONxe2x80x9d. Therein disclosed is an apparatus for controlling ink in a continuous ink jet printer. The apparatus comprises an ink delivery channel, a source of pressurized ink in communication with the ink delivery channel, and a nozzle having a bore, which opens into the ink delivery channel, from which a continuous stream of ink flows. A droplet generator inside the nozzle causes the ink stream to break up into a plurality of droplets at a position spaced from the nozzle. The droplets are deflected by heat (rather than by electrostatic charge) in the nozzle bore, from a heater having a selectively actuated section; i.e. a section associated with only a portion of the nozzle bore. Selective actuation of a particular heater section, at a particular portion of the nozzle bore produces what has been termed an asymmetrical application of heat to the stream. Alternately actuating the sections can serve to alternate the direction in which this asymmetrical heat is applied and thereby selectively deflects the ink droplets, inter alia, between a xe2x80x9cprintxe2x80x9d direction (onto a recording medium) and a xe2x80x9cnon-printxe2x80x9d direction (back into a xe2x80x9ccatcherxe2x80x9d).
Referring to FIG. 1, the application of heat causes deflection of ink drops 2, the magnitude of which depends upon several factors, e.g. the geometric and thermal properties of the nozzles, the pressure applied to, and the physical, chemical and thermal properties of the ink and, the flow pattern of the ink, prior to its emission from the nozzles. Deflected drops 2 impinge on a recording medium 19 while non-deflected drops 1 are passed from a gutter 17 to collection bottles and recycled. Alternatively, non-deflected drops 1 can impinge recording medium 19 while deflected drops 2 are collected by gutter 17. U.S. Pat. No. 6,079,821 discloses a system of this type.
The application of heat (for example, the asymmetric application of heat as disclosed by U.S. Pat. No. 6,079,821, etc.), as a means for deflecting continuous ink, has a number of advantages over electrostatic deflection. Electrostatic deflection of continuous streams of ink requires ink formulations having stringent specifications with respect to electrical conductivity. For example, conductivity control components are formulated into such ink. Those components may include soluble ionizable salts such as alkali metal and alkaline earth metal halides, nitrates, thiocyanates, acetates, propionates, and amine salts. These components are unnecessary for asymmetrical heat-deflection. Also, these conductive salt components are corrosive to metal parts of the printer and therefore require inclusion of corrosion inhibitors in the ink, which, in turn, must be sufficiently compatible with other formulated ink components that control for example, viscosity, conductivity, or the like. An advantage of heat over electrostatic deflection, was thought to be that thermal inks did not require such complex formulations and conductive components.
Nevertheless, continuous ink jet systems can accumulate contamination and trace metal ions from the atmosphere and internal parts as the continuous stream of ink recirculates. Additionally, ink jet systems utilizing heat can experience a problem called kogation from insoluble inorganic salts and carbon being deposited onto the surface of the nozzles can lead to improper operation of the print head. This can occur even in electrostatic systems if heated drop generators are used. Ink jet systems can also experience corrosion of printhead components from inorganic salts. Accordingly, inks that can be even more expensive than electrostatic inks, and which have dyes that are pretreated as in U.S. Pat. No. 5,755,861 by Fujioka et al. or U.S. Pat. No. 4,786,327 by Wenzel, or U.S. Pat. No. 5,069,718 by Kappele have been contemplated. These ion-exchange treatments of dyes used in drop-on-demand ink jet systems were done prior to addition of solvent vehicles such as glycols. However, neither corrosion inhibitors nor these ion-exchange pretreated inks having ion-exchanged dyes can provide protection from ink jet failure that stems from continuously accumulating contamination while recirculating the ink. An improvement, in continuous ink jet systems, that would inhibit contamination from recirculated ink would be a novel and welcomed advancement in the art, and has particularly surprising advantages in heated systems.
Therefore it is a principal object of the present invention to provide a method for removing trace metal ions while printing with a continuous ink jet system.
It is another object of the present invention to provide an improved continuous ink jet printer, particularly where heat is employed in the print heads, and an ink recirculation system which extends the life of the print heads.
This objective and others may be fulfilled by incorporating an ion-exchange resin bed into the ink recirculation system of a continuous ink jet printer, particularly one having a print head that uses heat (for example, asymmetric, symmetric, segmented heaters, etc.) to deflect the streams of ink droplets and/or to form the ink droplets. By continuously removing trace metal ions from the ink, and continuously reconstituting the ink, the clogging of nozzles, nozzle plate orifices, or ink channels in thermally controlled continuous ink jet print heads is substantially inhibited.
The apparatus of the invention removes dissolved, deleterious ions from the heated ink stream with an ion-exchange resin bed. Exchanging ink-deleterious ions for the ions originally bound to the resin does not hurt ink performance. That is, the latter are non-deleterious ions. The non-deleterious ion-exchange resins can be micro-reticular, macro-reticular, porous or macro-porous. Such resins can be selected from three broad types, i.e. anion exchange resins, cation exchange resins, and mixed-bed resins that can sequester both anions and cations. Both strong and weak ion-exchange resins may be useful and are well known in the art.